In frequently used code paths, such as controller actions, you should avoid using the https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpclient[HttpClient] directly and opt for https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory[one of the IHttpClientFactory-based mechanisms] instead. This way, you avoid wasting resources and creating performance overhead.

== Why is this an issue?

If a code path that creates and disposes of HttpClient objects is frequently used, then the following issues can occur: 

- Under heavy load, there's the risk of https://learn.microsoft.com/en-us/dotnet/fundamentals/networking/http/httpclient-guidelines#pooled-connections[running out of available sockets], leading to https://learn.microsoft.com/en-us/dotnet/api/system.net.sockets.socketexception[SocketException] errors. This is because each HttpClient instance uses a separate network connection, and there's a limit to the number of connections that can be opened simultaneously. Note that even after you dispose of an HttpClient https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests#issues-with-the-original-httpclient-class-available-in-net[its sockets are not immediately freed up].
- Each HttpClient has its own set of resources (like headers, base address, timeout, etc.) that must be managed. Creating a new HttpClient for every request means these resources are not being reused, leading to resource waste.
- You introduce a significant performance overhead when creating a new HttpClient for every HTTP request.

== How to fix it

The https://learn.microsoft.com/en-us/dotnet/api/system.net.http.ihttpclientfactory[`IHttpClientFactory`] was introduced in ASP.NET Core 2.1 to solve these problems. It handles pooling HTTP connections to optimize performance and reliability.

There are https://learn.microsoft.com/en-us/aspnet/core/fundamentals/http-requests#consumption-patterns[several ways] that you can use IHttpClientFactory in your application:

- https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory#basic-usage[Basic usage]
- https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory#named-clients[Named Clients]
- https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory#typed-clients[Typed Clients]
- https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory#generated-clients[Generated Clients]

Alternatively, you may cache the HttpClient in a singleton or a static field. You should be aware that by default, the HttpClient doesn't respect the DNS's Time To Live (TTL) settings. If the IP address associated with a domain name changes, HttpClient might still use the old, cached IP address, leading to failed requests.

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
[ApiController]
[Route("controller")]
public class FooController : Controller
{
    [HttpGet]
    public async Task<string> Foo()
    {
        using var client = new HttpClient();  // Noncompliant
        return await client.GetStringAsync(_url);
    }
}
----

==== Compliant solution

[source,csharp,diff-id=1,diff-type=compliant]
----
// File: Startup.cs
public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddHttpClient();
        // ...
    }
}

[ApiController]
[Route("controller")]
public class FooController : Controller
{
    private readonly IHttpClientFactory _clientFactory;

    public FooController(IHttpClientFactory clientFactory)
    {
        _clientFactory = clientFactory;
    }

    [HttpGet]
    public async Task<string> Foo()
    {
        using var client = _clientFactory.CreateClient(); // Compliant (Basic usage)
        return await client.GetStringAsync(_url);
    }
}
----

== Resources

=== Documentation

* S6420 - Client instances should not be recreated on each Azure Function invocation
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/api/system.net.http.ihttpclientfactory[IHttpClientFactory Interface]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpclient[HttpClient Class]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/core/extensions/httpclient-factory[IHttpClientFactory with .NET]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests[Use IHttpClientFactory to implement resilient HTTP requests]
* Microsoft Learn - https://learn.microsoft.com/en-us/aspnet/core/fundamentals/http-requests[Make HTTP requests using IHttpClientFactory in ASP.NET Core]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/fundamentals/networking/http/httpclient-guidelines[Guidelines for using HttpClient]

include::../rspecator.adoc[]